This invention relates to a distributor type fuel injection pump for internal combustion engines, and more particularly to improvements in the fuel injection timing characteristic of such pump within a low engine speed (rpm) range.
A conventional distributor type fuel injection pump for use in a diesel engine is generally provided with a load-responsive pressure adjusting device (hereinafter called "load timer") which is usually located in a centrifugal governor provided in the pump and serves to allow part of the fuel temporarily stored in the fuel suction chamber of the pump to flow into a zone under lower pressure in the pump in response to engine loads, i.e., injected fuel quantities. The drainage of part of the fuel in the suction chamber causes a decrease in the amount of advance of injection timing obtained by a fuel injection timing control device which is provided in the pump and is responsive to fuel pressure in the suction chamber, to retard the moment of injection of fuel into engine cylinders, thus leading to reduced combustion noise as well as reduced temperature in the engine cylinders, the latter of which in turn leads to smaller quantities of NOx produced.
On the other hand, the above-mentioned fuel injection timing control device includes a hydraulically actuatable member which is displaceable in response to a pressure being a function of the engine rpm to vary the moment of injection of the pump. This control device is conventionally provided with a cold starting device (CSD) which is responsive to the pressure being a function of the engine rpm to bias the hydraulically actuatable member in an injection timing advancing direction when the engine operates at speeds below a predetermined engine rpm, to thereby improve the startability of the engine in cold weather. According to this injection timing control device provided with such cold starting device, injection timing control is carried out with an hysteresis characteristic. That is, during the increase of the engine speed, a predetermined amount of injection timing advance is continuously obtained from the start of the engine until a predetermined engine rpm is reached and then a usual injection timing advance action is carried out after the predetermined engine rpm is reached. When the engine speed is decreasing, the timing advance is gradually decreased as the engine speed decreases from a high engine rpm range, and then the timing advance reaches a minimum value in a low engine rpm range including the idling rpm. Due to this injection timing control with an hysteresis characteristic, the engine can be free of knocking noise and murky smoke which would otherwise occur in idling or the like engine conditions.
When this injection timing control device constructed as above is used in combination with the aforesaid type of load timer, it sometimes fails to properly operate so that a predetermined amount of injection timing retard cannot be obtained within the above-mentioned low engine rpm range during decrease of the engine speed, since these two devices are arranged to be operated by a common hydraulic pressure, that is, the fuel pressure in the suction chamber of the fuel injection pump.